Thin p-type gallium nitride and aluminum gallium nitride electron-blocking layer free gallium nitride-based light emitting diodes

ABSTRACT

A light emitting diode (LED) having a p-type layer having a thickness of 100 nm or less, an n-type layer, and an active layer, positioned between the p-type layer and the n-type layer, for emitting light, wherein the LED does not include a separate electron blocking layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. Section 119(e) ofco-pending and commonly-assigned U.S. Provisional Application Ser. No.61/111,642 filed on Nov. 5, 2008, by Hong Zhong, Anurag Tyagi, James S.Speck, Steven P. DenBaars, and Shuji Nakamura, entitled “THIN P-TYPEGALLIUM NITRIDE AND ALUMINUM GALLIUM NITRIDE ELECTRON-BLOCKING LAYERFREE GALLIUM NITRIDE-BASED LIGHT EMITTING DIODES,” attorney's docketnumber 30794.291-US-P1 (2009-156), which application is incorporated byreference herein.

This application is related to the following co-pending andcommonly-assigned U.S. patent applications:

U.S. Utility patent application Ser. No. 12/370,479, filed on Feb. 12,2009, by Arpan Chakraborty, Benjamin A. Haskell, Stacia Keller, James S.Speck, Steven P. DenBaars, Shuji Nakamura and Umesh K. Mishra, entitled“FABRICATION OF NONPOLAR INDIUM GALLIUM NITRIDE THIN FILMS,HETEROSTRUCTURES AND DEVICES BY METALORGANIC CHEMICAL VAPOR DEPOSITION,”attorneys' docket no. 30794.117-US-C2, which application is acontinuation under 35 U.S.C. §120 of U.S. Utility patent applicationSer. No. 11/621,479, filed on Jan. 9, 2007, now U.S. Pat. No. 7,504,274issued Mar. 17, 2009, by Arpan Chakraborty, Benjamin A. Haskell, StaciaKeller, James S. Speck, Steven P. DenBaars, Shuji Nakamura and Umesh K.Mishra, entitled “FABRICATION OF NONPOLAR INDIUM GALLIUM NITRIDE THINFILMS, HETEROSTRUCTURES AND DEVICES BY METALORGANIC CHEMICAL VAPORDEPOSITION,” attorneys' docket number 30794.117-US-C1 (2004-495-3),which application is a continuation under 35 U.S.C. §120 of U.S. Utilitypatent application Ser. No. 11/123,805, filed on May 6, 2005, by ArpanChakraborty, Benjamin A. Haskell, Stacia Keller, James S. Speck, StevenP. DenBaars, Shuji Nakamura and Umesh K. Mishra, entitled “FABRICATIONOF NONPOLAR INDIUM GALLIUM NITRIDE THIN FILMS, HETEROSTRUCTURES ANDDEVICES BY METALORGANIC CHEMICAL VAPOR DEPOSITION,” attorneys' docketnumber 30794.117-US-U1 (2004-495-2), now U.S. Pat. No. 7,186,302, issuedon Mar. 6, 2007, which application claims the benefit under 35 U.S.C.§119(e) of U.S. Provisional Patent Application Ser. No. 60/569,749,filed on May 10, 2004, by Arpan Chakraborty, Benjamin A. Haskell, StaciaKeller, James S. Speck, Steven P. DenBaars, Shuji Nakamura and Umesh K.Mishra, entitled “FABRICATION OF NONPOLAR InGaN THIN FILMS,HETEROSTRUCTURES AND DEVICES BY METALORGANIC CHEMICAL VAPOR DEPOSITION,”attorneys' docket number 30794.117-US-P1 (2004-495-1);

U.S. Utility patent application Ser. No. 11/621,482, filed on Jan. 9,2007, by Troy J. Baker, Benjamin A. Haskell, Paul T. Fini, Steven P.DenBaars, James S. Speck, and Shuji Nakamura, entitled “TECHNIQUE FORTHE GROWTH OF PLANAR SEMI-POLAR GALLIUM NITRIDE,” attorneys' docketnumber 30794.128-US-C1 (2005-471-3), which application is a continuationunder 35 U.S.C. §120 of U.S. Utility patent application Ser. No.11/372,914, filed on Mar. 10, 2006, by Troy J. Baker, Benjamin A.Haskell, Paul T. Fini, Steven P. DenBaars, James S. Speck, and ShujiNakamura, entitled “TECHNIQUE FOR THE GROWTH OF PLANAR SEMI-POLARGALLIUM NITRIDE,” attorneys' docket number 30794.128-US-U1 (2005-471-2),now U.S. Pat. No. 7,220,324, issued on May 22, 2007, which applicationclaims the benefit under 35 U.S.C. §119(e) of U.S. Provisional PatentApplication Ser. No. 60/660,283, filed on Mar. 10, 2005, by Troy J.Baker, Benjamin A. Haskell, Paul T. Fini, Steven P. DenBaars, James S.Speck, and Shuji Nakamura, entitled “TECHNIQUE FOR THE GROWTH OF PLANARSEMI-POLAR GALLIUM NITRIDE,” attorneys' docket number 30794.128-US-P1(2005-471-1); and

U.S. Utility patent application Ser. No. 11/852,908, filed on Sep. 10,2007, by Michael D. Craven and James S. Speck, entitled “NON-POLARA-PLANE GALLIUM NITRIDE THIN FILMS GROWN BY METALORGANIC CHEMICAL VAPORDEPOSITION,” attorneys docket number 30794.245-US-I1 (2002-301-4), whichapplication is a continuation-in-part of:

U.S. Utility patent application Ser. No. 10/413,691, entitled “NON-POLARA-PLANE GALLIUM NITRIDE THIN FILMS GROWN BY METALORGANIC CHEMICAL VAPORDEPOSITION,” filed on Apr. 15, 2003, by Michael D. Craven and James S.Speck, attorneys docket number 30794.100-US-U1 (2002-294-2), whichapplication claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Patent Application Ser. No. 60/372,909, filed on Apr. 15,2002, by Michael D. Craven, Stacia Keller, Steven P. DenBaars, TalMargalith, James S. Speck, Shuji Nakamura, and Umesh K. Mishra, entitled“NON-POLAR GALLIUM NITRIDE BASED THIN FILMS AND HETEROSTRUCTUREMATERIALS,” attorneys docket number 30794.95-US-P1 (2002-294/301/303);and

U.S. Utility patent application Ser. No. 11/472,033, entitled “NON-POLAR(Al, B, In, Ga)N QUANTUM WELL AND HETEROSTRUCTURE MATERIALS ANDDEVICES,” filed on Jun. 21, 2006, by Michael D. Craven, Stacia Keller,Steven P. DenBaars, Tal Margalith, James S. Speck, Shuji Nakamura, andUmesh K. Mishra, attorneys docket number 30794.101-US-D1 (2002-301-3),which application is a divisional application and claims the benefitunder 35 U.S.C. §120 and §121 of U.S. Utility patent application Ser.No. 10/413,690, filed on Apr. 15, 2003, by Michael D. Craven et al.,entitled “NON-POLAR (Al, B, In, Ga)N QUANTUM WELL AND HETEROSTRUCTUREMATERIALS AND DEVICES,” attorney's docket number 30794.101-US-U1(2002-301-2), now U.S. Pat. No. 7,091,514, issued on Aug. 15, 2006,which application claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Patent Application Ser. No. 60/372,909, filed on Apr. 15,2002, by Michael D. Craven, Stacia Keller, Steven P. DenBaars, TalMargalith, James S. Speck, Shuji Nakamura, and Umesh K. Mishra, entitled“NON-POLAR GALLIUM NITRIDE BASED THIN FILMS AND HETEROSTRUCTUREMATERIALS,” attorneys docket number 30794.95-US-P1 (2002-294/301/303);

all of which applications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to light emitting diodes (LEDs) and in particularto high efficiency and high brightness LEDs for various lightingapplications, and methods of fabricating the same.

2. Description of the Related Art

(Note: This application references a number of different publications asindicated throughout the specification by one or more reference numberswithin brackets, e.g., [x]. A list of these different publicationsordered according to these reference numbers can be found below in thesection entitled “References.” Each of these publications isincorporated by reference herein.)

Gallium nitride (GaN)-based wide band gap semiconductor light emittingdiodes (LEDs) have been available for almost 15 years. The progress ofLED development has brought about great changes in LED technology, withthe realization of full-color LED displays, LED traffic signals, whiteLEDs and so on.

Highly efficient white LEDs have gained much interest as possiblereplacements for fluorescent lamps. For example, the luminous efficacyof white LEDs (130-150 lumens/watt [1]) already surpasses that ofordinary fluorescent lamps (75 lumens/watt.) Nevertheless, currentcommercially available wurtzite nitride-based LEDs are characterized bythe presence of polarization-related electric fields insidemulti-quantum wells (MQWs), for their [0001] c-polar growth orientation.The discontinuities in both spontaneous and piezoelectric polarizationat the heterointerfaces result in internal electric fields in quantumwells which cause carrier separation (quantum confined Stark effect(QCSE)) and reduce the radiative recombination rate within quantum wells[2-5]. To maintain a decent radiative recombination rate, c-polar lightemitting devices typically have thin (<3 nm) quantum wells [6-7].

To decrease these polarization-related effects, growing III-nitridedevices on the non-polar planes (viz, the (1-100) m-plane or the (11-20)a-plane) has been demonstrated [8-9]. Another approach to reduce, andpossibly eliminate, those effects is to grow III-nitride devices oncrystal planes that are inclined with respect to the c-direction, i.e.,semi-polar planes. These planes have reduced polarization discontinuityin heterostructures, compared with the c-plane III-nitride materials;and for semi-polar planes oriented ˜45° from the c-plane, there is nopolarization discontinuity in InGaN/GaN heterostructures [5]. Withreduced polarization-related electric fields inside the quantum wellregion, the electron and hole wavefunctions inside a semi-polar-orientedInGaN quantum well are expected to have more overlap (and thus lead to ahigher radiative efficiency) than in a c-polar oriented counterpart, fora given quantum well thickness. In other words, without worrying aboutthe detrimental effect on the radiative recombination rate, one canemploy thick quantum well designs in semi-polar LEDs. Devices grown ondifferent semi-polar planes (including (10-1-1), (10-1-3), (11-22)planes etc.) have been demonstrated and they exhibited greatly reducedpolarization-related electric fields [10-12]. The output powers of thoseheteroepitaxially-grown devices, however, suffer from the presence ofstacking faults and threading dislocations. Recently, with the advent ofhigh quality freestanding GaN substrates, high performance non-polar andsemi-polar LEDs with peak emission wavelengths ranging from 407 nm to513 nm on non-polar m-plane, semi-polar (10-1-1), and (11-22)freestanding GaN substrates have been reported [13-17]. Nonetheless, theoutput powers of those devices are still lower than that of typicalthe-state-of-the-art c-polar devices, which can be partly attributed tothe un-optimized LED epitaxial layer structure.

This invention presents novel semi-polar LED epitaxial layer structuresthat are expected to address this problem.

SUMMARY OF THE INVENTION

To overcome the limitations in the prior art described above, and toovercome other limitations that will become apparent upon reading andunderstanding the present invention, the present invention comprises ofa novel approach for producing a GaN-based semi-polar-oriented lightemitting diode (LED) that contains a thin (i.e. <100 nanometers (nm))p-type GaN layer and no AlGaN electron-blocking layer (EBL). In thepreferred embodiment, freestanding semi-polar (10-1-1) GaN is used asthe substrate for the LED. This invention offers several advantages overthe existing semi-polar LEDs. First of all, the thin p-type GaN layerand AlGaN EBL free designs can lower series-resistance and thus theoperating voltage of a GaN-based LED. Additionally, the thin p-type GaNlayer may reduce the absorption of light emitted from the quantum wellregion. Furthermore, the hole-carrier injection efficiency is likely tobenefit from the removal of the AlGaN EBL.

In one embodiment an LED, comprises a p-type layer having a thickness of100 nm or less; an n-type layer; and an active layer, positioned betweenthe p-type layer and the n-type layer, for emitting light; wherein thep-type layer, n-type layer and active layer are comprised of semi-polarnitride based material. For example, the p-type layer may have athickness of at most 50 nm. The LED may be III-nitride based but notcontain an AlGaN electron blocking layer, for example.

The active layer may comprise one or more quantum wells having athickness of 4 nm or more, wherein the active layer/quantum wells have athickness thick enough, and a composition, to capture and confineelectron-carriers in the active layer, thereby providing the function ofan electron blocking layer. The quantum wells may be InGaN quantumwells, for example.

The LED of the present invention may have a higher luminous efficiency,crystal quality, hole injection efficiency, lower series-resistance,operating voltage, and light absorption as compared to an LED comprisinga separate AlGaN blocking layer and thicker p-type layer.

The present invention further discloses a method for fabricating an LED,comprising depositing an n-type layer on a substrate; depositing anactive layer for emitting light on the n-type layer (e.g., growing oneor more InGaN quantum wells to a thickness greater than 4 nm and to athickness and with a composition that captures and confineselectron-carriers in the active layer, thereby providing the function ofan electron blocking layer); and depositing a p-type layer on the activelayer, to a thickness of 100 nm or less (e.g., to a thickness of at most50 nm); wherein the p-type layer, n-type layer and active layer arecomprised of semi-polar nitride based material.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawing in which like reference numbers representcorresponding parts throughout:

FIG. 1( a) is a cross-sectional schematic of an LED according to oneembodiment of the present invention.

FIG. 1( b) is a cross-sectional schematic of an example of an activeregion of the present invention.

FIG. 2 is a schematic cross-section of another example of an epitaxiallayer structure for a thin p-type GaN layer and AlGaN EBL-freesemi-polar LED, comprising an InGaN/GaN MQW or single quantum well(SQW).

FIG. 3 is a schematic cross-section of yet another example of an LEDaccording to the present invention.

FIG. 4 is a flowchart illustrating a method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Overview

The purpose of this invention is to generate semi-polar-orientedGaN-based LEDs with improved performance and manufacturability. Theproposed device will typically be used as an optical source for variousdisplay, illumination, and solid-state lighting applications.

The realization of thin p-type GaN and AlGaN EBL free LED structureswould potentially allow for multiple advances in the manufacturabilityof GaN-based LEDs (because this invention further simplifies LEDepitaxial structure) and yield devices with reduced voltage operationand higher crystal quality. The semi-polar GaN platforms are expected tooffer performance advantages such as devices with higher radiativerecombination rate and thus higher internal quantum efficiency. Theseadvantages will potentially lower the cost of a variety of commercialproducts.

Nomenclature

The term “(Al, Ga, In)N” or III-Nitride as used herein is intended to bebroadly construed to include respective nitrides of the single species,Al, Ga, and In, as well as binary, ternary and quaternary compositionsof such Group III metal species. Accordingly, the term (Al, Ga, In)Ncomprehends the compounds AN, GaN, and InN, as well as the ternarycompounds AlGaN, GaInN, and AlInN, and the quaternary compound AlGaInN,as species included in such nomenclature. When two or more of the (Ga,Al, In) component species are present, all possible compositions,including stoichiometric proportions as well as “off-stoichiometric”proportions (with respect to the relative mole fractions present of eachof the (Ga, Al, In) component species that are present in thecomposition), can be employed within the broad scope of the invention.Accordingly, it will be appreciated that the discussion of the inventionhereinafter in reference to GaN materials is applicable to the formationof various other (Al, Ga, In)N material species. Further, (Al,Ga,In)Nmaterials within the scope of the invention may further include minorquantities of dopants and/or other impurity or inclusional materials.

Technical Description

This invention is a semiconductor LED comprised of alloys such as, butnot limited to, (Ga, In, Al)N. In comparison to the other semi-polarLEDs reported thus far, whose p-type GaN thicknesses range from 150 nmto 200 nm [14-18], the proposed devices contain a thinner p-type GaNlayer (i.e. <100 nm and a typical value of 50 nm). Moreover, unlikepreviously reported semi-polar LEDs [14-18], the proposed devicescontain no AlGaN electron-blocking layer (EBL), which is typicallysandwiched between the quantum well region and the p-type GaN layer. Thedevice is grown and processed using well-established semiconductordevice fabrication techniques.

FIG. 1( a) illustrates a device 100 (e.g., LED) according to the presentinvention, comprising a p-type layer 102 having a thickness 104 of 100nm or less (e.g., at most 50 nm); an n-type layer 106; and an activeregion or layer 108, positioned between the p-type layer 102 and then-type layer 106, for emitting light. The active layer 108 typically,although not necessarily, has a thickness 110 thick enough, and acomposition, to capture and confine electron-carriers in the activelayer 108, thereby providing the function of an EBL. Thus, the presentinvention may integrate the EBL and active layer 108 as one layer 108providing both light emitting and electron blocking functionality. Thus,in one embodiment, the LED 100 does not include a separate EBL (distinctfrom layers 102, 106, 108) grown specifically for the sole purpose ofelectron blocking For example, the LED 100 typically does not contain anAlGaN blocking layer.

As a result, the LED may have a higher luminous efficiency, crystalquality, hole injection efficiency, lower series-resistance, operatingvoltage, and light absorption as compared to an LED comprising aseparate AlGaN blocking layer and thicker p-type layer.

The LED is typically III-Nitride based, e.g., layers 102, 106, and 108are III-Nitride (Al,In,Ga)N layers.

As shown in FIG. 1( b), the active layer 108 may comprise one or morequantum wells 112 having a thickness 114 of 4 nm or more, and acomposition (e.g., InGaN) to emit light. In addition, the active layer108 may comprise one or more quantum wells 112 having the compositionand the thickness 114 to capture and confine electron-carriers in thequantum well 112, thereby providing the function of an electron blockinglayer. The quantum wells 112 (e.g., InGaN) are typically sandwichedbetween a first quantum well barrier 116 (e.g., GaN) and a secondquantum well barrier 118 (e.g., GaN). The electron carriers typicallyflow to the active layer 108 and quantum wells 112, in response to apower source, and recombine with holes in the active layer 108 andquantum wells 112, to generate the light emitted by the device 100.

The LED 100 typically has a semi-polar or non-polar orientation 120, forexample, layers 102, 106, and 108 are grown along a non-polar orsemi-polar orientation 120, or cut, such that the interface 122 betweenthe active layer 108 and the n-type layer 106 and the interface 124between the active layer 108 and p-type layer 102 are a selectednon-polar or semi-polar plane. The interface 126 between the quantumwell 112 and first barrier 116 and the interface 128 between the quantumwell 112 and the second barrier 118 are also the selected non-polar orsemi-polar plane(s). The semi-polar or non non-polar orientation 120 istypically along the quantum confinement direction (the direction alongwhich the electrons and holes in the quantum well 112 are confined bythe energy barrier provided by the barrier layers 116, 118), so that theQCSE is reduced as compared to a c-plane device.

In one embodiment, all the layers, e.g., the p-type layer 102, n-typelayer 106, and active layer 108 are comprised of semi-polar nitridebased material.

FIG. 2 shows a typical epitaxial layer structure 200 for the proposeddevice. This is one embodiment of the invention. Other epitaxialstructures are possible and several alternative embodiments will besuggested in the following sections. As a first example, a device 200 isgrown on a semi-polar (10-1-1) freestanding GaN substrate 202, startingwith an unintentionally doped (UID) and/or n-type GaN layer 204,followed by a quantum well region 206 that may contain an InGaN/GaN SQWor MQWs, followed by a thin p-type GaN layer 208. Given the device'ssemi-polar orientation, the individual quantum well thickness could bethicker than what is typically used for c-polar devices (i.e. >4 nm).The (10-1-1) freestanding GaN substrate 202 has a surface 210 upon whichlayer 204 is epitaxially grown that is a semi-polar (10-1-1) plane.

FIG. 3 shows a second example of a semi-polar LED device 300 with a thinp-type GaN layer 302, and no AlGaN EBL, that was grown by metal organicchemical vapor deposition (MOCVD) on a freestanding semi-polar (10-1-1)GaN substrate 304. The LED epitaxial layer structure includes a 1.0 μmthick silicon-doped n-type GaN 306, active region comprising a 3 nmthick quantum well 308 and 30 nm thick UID GaN barriers 310, 312, 30 nmthick magnesium (Mg)-doped p-type GaN layer 302 and a 20 nm thickheavily Mg-doped p⁺-type GaN contact layer 314. Following the growth,LEDs were fabricated and packaged on silver headers. 250 nm thickindium-tin-oxide (ITO) was used as the p-type GaN contacts 316 and Ti/Auwas used as the p-type and n-type electrodes 318, 320. A representativeLED 300 was tested on a bare header (without any encapsulation andwithout any intentional light extraction scheme) in an integratingsphere under Direct Current (DC) operation. At 20 milliamps (mA) drivecurrent, a peak-emission wavelength of 444 nm and an output power of15.2 milliwatts (mW) was measured. In comparison, the output power ofthe best semi-polar blue LED described in the literature [1,5], on abare silver header (without any encapsulation and without anyintentional light extraction scheme), and measured in an integratingsphere, was only 11.6 mW at 20 mA under DC operation. It should be notedthat except for the thick (200 nm) p-type GaN layer and AlGaN EBL, thissemi-polar LED (described in [1,5]) is almost identical to the secondexample of the present invention, including the peak EL emissionwavelength, crystal orientation, quantum well thickness, and ITOp-contact thickness.

The implementation of the proposed epitaxial structure to producevarious categories of GaN-based LEDs is the core of the invention. Thisepitaxial structure may be fabricated into a variety of Thin P-type GaNlayer and AlGaN EBL Free semi-polar (TPAF) LEDs using standardsemiconductor processing techniques.

There are multiple applications and variations on the invention, asdiscussed in the sections above and below.

Process Steps

FIG. 4 illustrates a method for fabricating an LED, comprising thefollowing steps.

Block 400 represents depositing (e.g., growing) a layer having a firstconductivity type (e.g., n-type layer) on a substrate, e.g., on anon-polar or semi-polar substrate or on a non-polar or semi-polar planeof a substrate, or on a substrate that supports semi-polar or non-polargrowth. The n-type layer may be grown, for example, along a non-polar orsemi-polar orientation of III-Nitride.

Block 402 represents depositing (e.g., growing) an active layer orregion, for emitting light, on the layer of block 400 having the firstconductivity type. In one example, depositing the active layer comprisesgrowing one or more quantum wells having a thickness of 4 nm or more.Furthermore, the method typically, although not necessarily, comprisesdepositing the active layer to a thickness and with a composition thatcaptures and confines electron-carriers, thereby providing the functionof an electron blocking layer. For example, depositing the active layermay comprise growing one or more quantum wells having the thickness andcomposition that captures and confines electron-carriers. The quantumwells may be grown as InGaN quantum wells.

Thus, a separate electron blocking layer, such as an AlGaN blockinglayer, need not be included in the LED.

The active layer is typically along a non-polar or semi-polarorientation of III-Nitride, on a non-polar or semi-polar plane of thelayer having the first conductivity type grown in block 400.

Block 404 represents depositing (e.g., growing) a layer having a secondconductivity type (e.g., p-type layer) and a thickness of 100 nm orless, on the active layer of block 402. In one example, the stepcomprises depositing the p-type layer to a thickness of at most 50 nm.The p-type layer may be grown, for example, along a non-polar orsemi-polar orientation of III-Nitride.

Block 406 represents the end result of the method, a device such as anLED. The p-type layer, the active layer, and the n-type layer aretypically III-nitride and the LED typically does not contain an AlGaNelectron blocking layer, for example. All layers, e.g., the p-typelayer, n-type layer and active layer, may be comprised of semi-polarnitride based material. The depositing steps (a), (b), and (c) mayresult in a higher luminous efficiency, crystal quality, hole injectionefficiency, lower series-resistance, operating voltage, and lightabsorption as compared to an LED comprising a separate AlGaN blockinglayer and thicker p-type layer.

Possible Modifications

Growth of TPAF LEDs may also be practiced on (Ga, In, Al)N crystalorientations other than the semi-polar (10-1-1) plane, including but notlimited to, all other semi-polar planes, and non-polar a-planes andm-planes. The term “semi-polar plane” can be used to refer to any planethat cannot be classified as c-plane, a-plane, or m-plane. Incrystallographic terms, a semi-polar plane would be any plane that hasat least two nonzero h, i, or k Miller indices and a nonzero 1 Millerindex. Non-polar m-planes and a-planes refer to (10-10) and (11-20)planes, respectively.

The preferred embodiment presented above discussed (Ga,Al,In)N thinfilms, heterostructures, and devices grown on a free-standing semi-polarnitride wafer having a composition lattice matched to the structure tobe grown. Freestanding semi-polar nitride wafers may be created byremoving a foreign substrate from a thick semi-polar nitride layer, bysawing a bulk nitride ingot or boule into individual semi-polar nitridewafers, or by any other possible crystal growth or wafer manufacturingtechnique. The scope of this invention includes the growth andfabrication of semi-polar (Ga,Al,In)N thin films, heterostructures, anddevices on all possible freestanding semi-polar nitride wafers createdby all possible crystal growth methods and wafer manufacturingtechniques. The substrate may also be thinned and/or polished and/orremoved in some instances.

Likewise, the (Ga,Al,In)N thin films, heterostructures, and devicesdiscussed above could be grown on a freestanding non-polar nitride waferhaving a composition lattice matched to the structure to be grown.Freestanding non-polar nitride wafers may be created by removing aforeign substrate from a thick non-polar nitride layer, by sawing a bulknitride ingot or boule into individual non-polar nitride wafers, or byany other possible crystal growth or wafer manufacturing technique. Thescope of this invention includes the growth and fabrication of non-polar(Ga,Al,In)N thin films, heterostructures, and devices on all possiblefreestanding non-polar nitride wafers created by all possible crystalgrowth methods and wafer manufacturing techniques. The substrate mayalso be thinned and/or polished in some instances.

Moreover, foreign substrates other than freestanding GaN could be usedfor semi-polar or non-polar template growth. The scope of this inventionincludes the growth and fabrication of semi-polar and non-polar(Ga,Al,In)N thin films, heterostructures, and devices on all possiblecrystallographic orientations of all possible substrates. Thesesubstrates include, but are not limited to, silicon carbide, galliumnitride, silicon, zinc oxide, boron nitride, lithium aluminate, lithiumniobate, germanium, aluminum nitride, lithium gallate, partiallysubstituted spinels, and quaternary tetragonal oxides sharing theγ-LiAlO₂ structure.

The semi-polar (Ga,Al,In)N devices described above were grown onfreestanding GaN wafers. However, the scope of this invention alsocovers non-polar or semi-polar (Ga,Al,In)N devices grown on non-polar orsemi-polar epitaxial laterally overgrown (ELO) templates. The ELOtechnique is a method of reducing the density of threading dislocations(TD) in subsequent epitaxial layers. Reducing the TD density leads toimprovements in device performance.

Variations in (Ga,In,Al)N quantum well and heterostructure design arepossible without departing from the scope of the present invention. Forexample, low aluminum composition AlGaN layers (Al_(x)GaN_(1-x)N,0≦x≦0.5), as well as AlInGaN quaternary layers, could be used as quantumwell barriers. Moreover, the specific thickness and composition of thelayers, the number of quantum wells grown, and the inclusion arevariables inherent to particular device designs and may be used inalternative embodiments of the present invention.

This invention may also be used to produce TPAF GaN-based LEDs that donot contain quantum wells. One example is an LED structure containing aGaN/InGaN double hetero structure.

Advantages and Improvements

This invention offers several advantages over the existing semi-polarLEDs. A thin p-type GaN and AlGaN-EBL-free semi-polar LED is expected tohave a higher luminous efficacy. This is because the thin p-type GaNlayer and absence of AlGaN EBL can lower series-resistance and thus theoperating voltage of a GaN-based LED. Additionally, the thin p-type GaNlayer may reduce the absorption of light emitted from the quantum wellregion. Furthermore, the hole-carrier injection efficiency is likely tobenefit from the removal of the AlGaN EBL, since while AlGaN EBLenhances capture and confinement of the electron-carriers in the quantumwell active region, it also obstructs hole-carrier injection [19].Instead of the AlGaN EBL, thick quantum well designs facilitated by thesemi-polar growth orientation can be utilized to help the capture andconfinement of the electron-carriers (a thick quantum well is expectedto be more efficient in capturing and confining electron-carriers[20-22]).

The implementation of the present invention is also expected to resultin higher LED crystal quality. This is because a thin p-type GaN layer(which is preferably grown at a much higher temperature than the InGaNquantum well growth temperature) requires a shorter growth time, whichin turn shortens the high temperature exposure for the InGaN quantumwell region, alleviating the possible deterioration of InGaN quantumwell region due to high temperature. Furthermore, the absence of AlGaNEBL layer is expected to improve the overall epitaxial layer crystalquality since this AlGaN layer is typically grown at a temperaturearound the InGaN quantum well growth temperature to avoid dissociationof InGaN quantum well. AlGaN grown at this temperature, which is notdesirable for high quality AlGaN growth, is likely to suffer from poorcrystal quality, in particular, high defect density. It should be notedthat since the proposed devices have a greatly simplified epitaxialstructure as compared to existing devices, the present invention canfurther improve the manufacturability of LEDs.

The concept of employing thin a p-type GaN layer in asemi-polar-oriented GaN-based LED to reduce its series-resistance, lightabsorption and better preserve the quality of InGaN quantum well isbelieved to be new. The concept of a semi-polar-oriented GaN-based LEDcontaining no AlGaN electron-blocking layer to improve hole-carrierinjection and the crystal quality of the LED epitaxial layers is alsobelieved to be new.

REFERENCES

The following references are incorporated by reference herein.

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CONCLUSION

This concludes the description of the preferred embodiment of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

1. A light emitting diode (LED), comprising: a p-type layer having athickness of 100 nm or less; an n-type layer; and an active layerpositioned between the p-type layer and the n-type layer, wherein thep-type layer, the n-type layer and the active layer are comprised ofsemi-polar nitride based material.
 2. The LED of claim 1, wherein thep-type layer has a thickness of at most 50 nm.
 3. The LED of claim 1,wherein the LED is III-nitride based and does not contain an AlGaNelectron blocking layer.
 4. The LED of claim 1, wherein the active layercomprises one or more quantum wells having a thickness of 4 nm or more.5. The LED of claim 1, wherein the active layer has a thickness thickenough, and a composition, to capture and confine electron-carriers inthe active layer, thereby providing the function of an electron blockinglayer.
 6. The LED of claim 5, wherein the active layer comprises one ormore quantum wells having the thickness.
 7. The LED of claim 6, whereinthe quantum wells are InGaN quantum wells.
 8. The LED of claim 1, havinga higher luminous efficiency, crystal quality, hole injectionefficiency, lower series-resistance, operating voltage, and lightabsorption as compared to an LED comprising a separate AlGaN blockinglayer and thicker p-type layer.
 9. A method for fabricating a lightemitting diode (LED), comprising: (a) depositing an n-type layer on asubstrate; (b) depositing an active layer, for emitting light, on then-type layer; and (c) depositing a p-type layer on the active layer, toa thickness of 100 nm or less; wherein the p-type layer, n-type layerand active layer are comprised of semi-polar nitride based material. 10.The method of claim 9, further comprising depositing the p-type layer toa thickness of at most 50 nm.
 11. The method of claim 9, wherein thep-type layer, the active layer, and the n-type layer are III-nitride andthe LED does not contain an AlGaN electron blocking layer.
 12. Themethod of claim 9, wherein depositing the active layer comprises growingone or more quantum wells having a thickness of 4 nm or more.
 13. Themethod of claim 9, further comprising depositing the active layer to athickness and with a composition that captures and confineselectron-carriers in the active layer, thereby providing the function ofan electron blocking layer.
 14. The method of claim 13, whereindepositing the active layer comprises growing one or more quantum wellshaving the thickness and composition.
 15. The method of claim 14,wherein the quantum wells are grown as InGaN quantum wells.
 16. Themethod of claim 1, wherein the depositing steps (a), (b), and (c) resultin a higher luminous efficiency, crystal quality, hole injectionefficiency, lower series-resistance, operating voltage, and lightabsorption as compared to an LED comprising separate AlGaN blockinglayer and thicker p-type layer.
 17. The method of claim 9, wherein thedepositing of steps (a),(b) and (c) further comprises growing along asemi-polar orientation.